The Henderson-Hasselbalch equation describes the chemical composition of a buffer as a function of pH. With the Henderson-Hasselbalch equation,

pH = pKa + log {[A^-]/[HA]} (see also, Equation 2.10),

where [HA] is the concentration of the undissociated weak acid and [A^-] is the concentration of the conjugate base of [HA], one can predict the pH of solutions of weak acids/weak bases or make buffers of desired pHs readily. This is a very simple equation of four variables, pH, pKa, [A^-], and [HA].

To solve the equation, you need three of the variables. The most common problem students have in solving the Henderson-Hasselbalch equation is understanding what they have been given. Consider the following problem:

What is the pH of a buffer mixture containing 1M acetic acid and 0.5M sodium acetate?

Answer - The concentration of acetic acid (1M) corresponds to the [HA] term and the concentration of acetate (from sodium acetate) (0.5M) corresponds to the [A^-] term. From Table 2.6, the pKa of acetic acid is 4.76.

Thus, pH = 4.76 + log ([0.5]/[1.0] ) = 4.46.

See another example here.